John Deere stx38 - Manual (Page 74)

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4/23/97
REPAIR ENGINE
ANALYZE PISTON RING WEAR
Rings of the wrong size or rings having improper end
gap will not conform to the shape of the cylinder. This
results in high oil consumption and excessive blowby.
Ring end gaps should be staggered on the piston
during installation. End gaps in alignment can also
cause oil consumption and blowby.
Light scuffing or scoring (A) of both rings and piston
occurs when unusually high friction and combustion
temperatures approach the melting point of the piston
material.
When this condition exists, it is due to one or more of
the following probable causes:
1. Dirty cooling shroud and cylinder head.
2. Lack of cylinder lubrication.
3. Improper combustion.
4. Wrong bearing or piston clearance.
5. Too much oil in crankcase causing fluid friction.
The engine operating at abnormally high temperatures
may cause varnish, lacquer, or carbon deposits (B) to
form in the piston grooves making the rings stick.
When this happens, excessive oil consumption and
blowby will occur.
Engine overheating and ring sticking is usually caused
by one or more of the following:
1. Overloading.
2. Incorrect ignition timing.
3. Lean fuel mixture.
4. Dirty cooling fins.
5. Incorrect oil.
6. Low oil supply.
7. Stale fuel.
Vertical scratches (C) across the piston and rings are
due to an abrasive in the engine. Abrasives may be
airborne, may have been left in the engine during
overhaul or may be loose lead and carbon deposits.
When this condition exists, check for one or more of
the following:
1. Damaged, collapsed, or improperly installed air
filter.
2. Loose connection or damaged gasket between air
cleaner and carburetor.
3. Air leak around carburetor-to-cylinder block gasket.
4. Air leakage around throttle shaft.
5. Failure to properly clean cylinder bore after
reconditioning engine.
Abrasive particles in engine oil cause scratches on side
rails (A) of oil control ring. If inner spacer (B) of oil
control ring is worn or distorted it may cause:
• Increased or high oil consumption.
• Increased deposits in combustion chamber.
• Sticking compression rings.
ANALYZE PISTON WEAR
DETONATION—
Detonation is uncontrolled combustion (A) caused by
excessive temperature and pressure in the combustion
chamber. Commonly called carbon knock, spark
knock, or timing knock; detonation occurs when the
extremely hot, over-compressed, fuel-air mixture
ignites spontaneously—causing a second burn point
during the normal ignition cycle. This second burn then
collides with the spark plug induced burn (B), causing a
loud explosion and extreme shockwave.
M29943
A
M29944
C
B
B
A
M38101
A
M58335
B